Conventional silencers of the type in which the silencer is inserted into the flow of gas to attenuate noise traveling in the gas stream have generally relied upon viscous friction in the pores of a cavity filler material.
A conventional silencer typically includes a duct member within which is positioned one or more silencer elements consisting of a perforated facing plate behind which is positioned a filler material, such as foam, rockwool, fiberglass or other fibrous acoustically absorptive bulk material. The filler may be referred to as packing.
Because these packed duct silencers rely on absorption by the packing, the perforated facing sheet is designed to provide optimum sound access from the flow passage to the packing material. Face sheet open face area in these silencers are typically 20% and more.
The use of packing to absorb acoustical noise introduces problems in many applications. The packing tends to erode under high velocity conditions; the packing may absorb toxic or flammable substances or microorganisms; the packing is subject to attack by chemicals; and in the event of fire, some otherwise desirable packings may provide fuel or produce toxic gases.
It has been known for nearly thirty years that, by using face sheets with suitable acoustic flow resistance in lieu of conventional perforated face sheets, broad band acoustical absorption could be obtained without the use of packing.
In order to overcome packing problems, silencers have been designed in which the required acoustic resistance was provided by thin resistive sheets rather than by packing. The resistive sheets of these constructions have been structurally self-supporting sintered materials of laminates of fabrics (metals, glass or synthetic), felts (metals, synthetic or organic) or sintered materials (metal or ceramics)-typically supported on a structural perforated sheet. These silencers have found very limited use due to their high cost.